It has been predicted that demand for airplanes will be increased to be twice of the current for airframes and 3.5 times of the current for engines within the next two decades. Improving fuel consumption of such sharply-increasing airplanes remarkably contributes to global environment in view of energy-saving and CO2 reduction. Accordingly, development of various technologies has been vigorously performed in view of lightening airframes and improving engine output efficiency.
On the other hand, as a matter of course, airframes and engines for airplanes require extremely high safety performance as prerequisite. When designing respective structural parts, lightening has to be achieved while sufficiently maintaining strength and stiffness. Accordingly, light and high-strength material has been widely utilized. The above is similar to steam turbine engines which are utilized in thermal power stations.
In particular, for engine parts, it is absolutely necessary to increase combustion temperature for improving combustion efficiency. Accordingly, temperature of rotating parts of a jet engine reaches one thousand and several hundred degrees Celsius and rotation thereof reaches high speed as being several thousand rpm. Thus, it is required to adopt shape and material of parts which endure extremely large centrifugal force under high temperature conditions. Accordingly, high-strength heat-resistant alloy such as titanium alloy and nickel-base alloy has been adopted as material being superior in heat-resistant properties and corrosion-resistant properties in addition to being light with high strength.
Optimization-based design with structural analysis has been performed for designing such engine parts and the like. It has been required to perform machining such as welding, casting, forging, cutting, grinding and polishing at designed high accuracy. In particular, regarding cutting machining which largely influences part performance such as rotation accuracy, a feature of high-strength heat-resistant alloy itself that strength is less subject to decreasing even at high temperature has been a large obstacle for improving machining accuracy and machining efficiency. Such high-strength heat-resistant alloy is called difficult-to-cut material.
Here, when cutting temperature reaches high temperature, there may be supposed a case that strength of a work made of high-strength heat-resistant alloy exceeds strength of a cutting tool. Accordingly, there may be a case that a cutting edge is damaged and machining cannot be performed while machinability is drastically worsened.
With such a background, it has been strongly desired to develop a method and an apparatus to cut high-strength heat-resistant alloy called difficult-to-cut material such as titanium alloy and nickel-base alloy at high efficiency while reducing abrasion of a cutting tool.
Specific examples include a disc and blades of a turbine utilized for a jet engine as in FIGS. 1A and 1B. As can be seen from a sectional view of the disc, arc-like groove machining is required to be performed against a disc-shaped work as being hatched with broken lines. Traditionally, for performing turning of such an arc-like groove, machining methods such as machining with a turning tool, machining with an end mill, machining with a milling cutter and machining with a side cutter have been utilized and appropriately adopted corresponding to work material, dimensions of a machining groove, required machining accuracy or the like. However, when performing machining a work made of difficult-to-cut material particularly such as titanium alloy and nickel-base alloy, following problems have occurred respectively.
In the case of utilizing a turning tool, since a part of a cutting edge is continuously contacted to a work during machining, there may be a case that some errors occur at machining dimensions owing to tool abrasion and a case that machining cannot be performed owing to frequent occurrence of tool breakage as a tool is instantaneously heated and worn.
In the case of utilizing an end mill, since a cutting edge formed at a side face of a tool intermittently cuts a work, a part of the side face of the tool is not contacted. However, since a contacting arc between the tool and work is long, heating thereby cannot be avoided. Further, since a bottom face is continuously contacted, heating is apt to occur from the position thereof. In addition, since chips are accumulated to the inside of a groove formed by end mill machining, biting by the tool frequently occurs.
In the case of utilizing a milling cutter, there arise physical restrictions for machinable groove depth owing to interference of an outer face of a tool against a work end face or a groove outer wall, as illustrated in FIG. 2.
In the case of machining by utilizing a turning tool or an end mill, a part of a cutting edge is continuously contacted to a work. However, in the case of utilizing a side cutter, since cutting edges are arranged at the outer circumference of the side cutter at predetermined intervals, cutting is intermittently performed as illustrated in FIG. 3. Since the tool is cooled down during non-cutting duration, tool heating can be prevented compared to continuous cutting. It is advantageous in view of elongation of tool lifetime thereby.
In machining with a side cutter, depending on shape of a tool, the tool is forwarded in the cut direction while maintaining a rotating shaft of the tool to be horizontal against a machining face. FIGS. 4 to 6 illustrate how a groove is formed by actual machining in a case that machining is performed against a work W by utilizing a side cutter as target machining shape being a circumferential groove at predetermined radius with a vertical side wall.
Here, in a side cutter 1, cutting edges 4 are arranged at even intervals on an outer circumference of a disc 3 of which center is matched with the center of a shaft 2. In FIGS. 4A and 4B, the shaft 2 is lowered as being maintained continuously in parallel to the work W rotating in the counterclockwise direction. As contacting to the end face of the work W, cutting of the circumferential groove is started. Cutting machining is continued until the shaft 2 is to be at groove depth of the machining target. Here, the disc 3 is rotated on the shaft 2 in the counterclockwise direction as viewing from the opposite side to the shaft 2 in FIG. 4B.
FIG. 4A is a plane view illustrating a state that the cutting edge 4 enters into the work most deeply. A cutting start point is a position at which contacting to the work W is started at the most upstream side as viewing from the rotation direction of the disc 3. Then, a cutting finish point is a position being an exit from the work W at the most downstream side, as viewing from the rotation direction of the disc 3, of a cutting edge trajectory after passing through a most deeply entering position in the work. In the following, the most deeply entering position in the work is called the deepest point of the cutting edge trajectory. The groove is gradually formed as the deepest point being on an inner diameter and the cutting start point and the cutting finish point being on an outer diameter as viewing the end face of the work W from the plane.
Here, the groove is not formed in parallel to the center line of the work W in radially sectioned shape. That is, in FIGS. 4A and 4B, a kerf on the end face of the work W is to be a line connecting the cutting start point and the cutting finish point. When viewing FIG. 4B as a side view, the cutting edge 4 passes through the inside of the work W being on a trajectory as a chord connecting the cutting start point and the cutting finish point having the midpoint of the chord being at the deepest point.
Here, the groove machining is performed until groove depth of the machining target is obtained as rotating the work W on the center point O while maintaining the shaft 2 continuously in parallel to the work W. Consequently, the sectional shape of the groove is to be determined with one turn of the work W on the center O against a cutting edge arc face of which chord is the line connecting the cutting start point and the cutting finish point as the midpoint of the chord being the deepest point within the work W.
Accordingly, in the case that machining of groove shape is performed at the end face of the rotating work by utilizing the side cutter 1, when the machining is performed so that the deepest point of the cutting edge trajectory is matched with the groove arc trajectory, a point on a tool trajectory projected on the machining face is to be apart from the arc curve of the groove toward the outer side of the groove by δ in a direction of an axis (hereinafter, the axis is defined as x-axis) which is obtained as projecting, on the work face, a straight line from the deepest point of the cutting edge 4 in the work to the center axis of the work W, as illustrated in FIG. 4A. Here, in the plane view, x-axis is matched with the shaft 2 of the side cutter 1 as being fixed regardless of the work rotation during machining.
Currently, there are two types of coordinate systems for NC machine tools as follows.
(1) Machine Coordinate System
A coordinate system indicating a structure of machine tool itself in which the original point position and directions of x, y, z axes are not varied even when a rotating table having a work mounted thereon is rotated.
(2) Table Coordinate System
A coordinate system in which the original point position is circumferentially moved and directions of x, y, z axes are varied when a rotating table having a work mounted thereon is rotated as being a coordinate system virtually set in numerical control (NC).
Among the above, the machine coordinate system of (1) is adopted in the present invention. Accordingly, the original point is continuously located at the same position and the x-axis direction is not varied regardless of the work rotation.
Here, δ denotes deviation occurring in the x-axis direction between the projected tool trajectory at each point on the tool trajectory and the arc curve of the groove being the target shape as tracing the projected tool trajectory from the cutting start point to the deepest point or from the deepest point to the cutting finish point. Difference between the tool trajectory and the groove arc at the cutting start point is denoted by δs and difference between the tool trajectory and the groove arc at the cutting finish point is denoted by δf.
The deviation δ is to be the maximum value at the cutting start point δs and the cutting finish point δf. Then, the larger the cutting edge radius of the side cutter 1 against the machining target radius is or the larger the groove depth of the machining target is, the larger the deviation δ becomes.
In this manner, the projected trajectory of the cutting edge is to be apart outside from the groove arc curve. Further, the entering depth of the cutting edge becomes deep from zero to the deepest point in an arc-like manner from the cutting start point to the deepest point. On the contrary, the entering depth of the cutting edge becomes shallow in an arc-like manner from the deepest point to the cutting finish point. Accordingly, as illustrated in FIG. 5, at the time when the machining is completed, the sectional shape of the machined groove has a curve-face-like bank occurring at an outer wall part thereof so that groove width is lessened in the depth direction.
In the following, the reason of the bank occurrence will be described further in detail with reference to FIG. 6.
In FIG. 6, numerals are used only in this drawing. Here, it is assumed that the machining target shape of the groove is a true circle having a side face perpendicular to a work. The trajectory of the side cutter is denoted by 1, the groove arc on the work face of the machining target shape is denoted by 2, the trajectory of the groove arc of the machining target shape at arbitrary depth from the work face is denoted by 3 to 5, and the trajectory of the groove arc at the deepest point of the machining target shape is denoted by 6.
Here, a point on the face of the work W on which the deepest point 22 is projected is assumed to be an original point O′, an axis oriented toward the work center axis from the original point O′ in parallel to the work face (in other words, an axis obtained by projecting the tool rotating shaft on the work face) is assumed to be x-axis, an axis being perpendicular to x-axis on the horizontal plane is assumed to be y-axis, and an axis being vertically perpendicular to x-axis is assumed to be z-axis.
Here, arbitrary points on the groove arc trajectory of the machining target shape respectively at certain depth are denoted by 7 to 10. Further, points on the cutting edge trajectory 1 at positions corresponding in the y-axis direction to the respective points 7 to 10 on a plane perpendicular to z-axis are denoted by 11 to 14. Then, line segments respectively connecting 7 with 11, 8 with 12, 9 with 13, and 10 with 14 denote the deviation δ in the x-axis direction between the abovementioned cutting edge trajectory and the groove arc of the machining target shape. In particular, length of the line segment connecting 7 with 11 is to be the deviation of between the cutting edge trajectory and the groove arc of the machining target shape at the cutting finish point.
Next, arcs 15 to 18 passing through the points 11 to 14 are drawn around the center axis of the work W in a section being parallel to the work W face, that is, in a section being perpendicular to z-axis. Then, the point 7 to 10 on the groove arc of the machining target shape are moved respectively along the groove arcs 2 to 5 to the point O′ to 22 on z-axis which is parallel to the work center axis and which passes through the point O′, that is, “y=0”.
Similarly, the points 11 to 14 are moved respectively along arcs 15 to 18 to points 23 to 26 on the x-z plane as being “y=0”. Then, a curve line connecting the points 23, 26 and 22 is to be a bank curve line at the outer wall occurring when the groove is machined with the side cutter type. The above becomes an error against the groove arc of the machining target shape.
In this manner, with the machining method to arrange a side cutter horizontally against a machining face, it is difficult to accurately finish arbitrary groove shape owing to influence of a bank. Extent thereof is to be related to a ratio of a diameter of a tool against a groove diameter. In general, the larger the diameter of a used tool is, the larger a bank occurs at an outer wall of the groove. Meanwhile, in a case with a tool of a small diameter, although extent of a bank occurring at the outer wall is small, a deep groove cannot be machined owing to the small diameter of the tool as being disadvantageous in machining efficiency and tool heating as well.
In addition, as described above, the shaft 2 of the tool 1 is lowered to be close to the work W as being continuously maintained in parallel. Accordingly, as illustrated in FIG. 7, with the work W having a protruded portion at the front side of a machining groove and the like, there occurs interference between the shaft 2 and the protruded portion causing a problem that it is impossible to machine a groove having predetermined depth or deeper.